For general photographic applications, it has been commonly accepted that a practical and economical zoom lens construction is one in which a first unit is used for focusing and two middle units are used for zooming and compensating. Such constructions also include a rearmost unit which contains an iris diaphragm which remains stationary during zooming. This type of lens construction has been used for most telephoto zoom lenses for 35 millimeter single-lens-reflex (SLR) cameras. The construction has provided fixed relative apertures (f-numbers) during zooming and the mechanical convenience of a fixed stop location.
However, with zoom lens ranges extended to include the wide angle region, the consequences of a fixed rear unit were found to be too severe. In particular, the front unit became large and thus heavy and expensive. Also, aberrations became excessive, thus requiring an increased number of lens elements. To overcome these problems, zoom lens systems were developed in which the aperture stop, and thus the exit pupil, moved during zooming. These systems were able to achieve a focal length range extending from about 0.8 to about 1.6 of the film diagonal while still having both a small diameter and a short length. In practice, the 0.8 to 1.6 range was adequate for most applications.
In a 35 millimeter SLR camera, the user views the object to be photographed by means of a viewing system, which includes a movable mirror that initially resides in the optical path following the zoom lens system to divert the image of the object to a viewing screen. This mirror is removed from the optical path prior to exposing the film. By texturing the viewing screen so that it produces a diffuse image, changes in the location of the exit pupil of the zoom lens system can be made effectively imperceptible to the user. Accordingly, for such applications, there is no need to fix the exit pupil location during zooming.
Instant photography cameras or other moderately large image formats impose requirements on zoom lens systems which are more difficult to satisfy than those imposed by 35 millimeter SLR cameras. For example, instant photography cameras use a film format substantially larger than that employed in a 35 millimeter camera, e.g., a diagonal greater than about 90 millimeters as opposed to a diagonal of about 42 millimeters as in conventional 35 millimeter film. If a conventional 35 millimeter SLR viewing system were to be used with such an instant photography camera, the result would be an impractically large camera body.
Consequently, viewing systems have been developed for instant photography cameras in which optical components, such as Fresnel mirrors, are introduced into the optical path between the taking lens and the viewing system's eye lens in order to reduce the size of the image presented to the eye lens. These viewing systems form an image at infinity which can be easily viewed by the user. Also, they image the exit pupil of the taking lens into the user's eye. As a result, the location of the exit pupil of a zoom lens system used with such a viewing system needs to remain substantially fixed with zooming so that the ultimate exit pupil of the zoom lens/viewing system combination remains at the user's eye throughout the zooming range.
As described below, in accordance with the invention, a fixed exit pupil is achieved by means of 1) a fixed aperture stop location for the zoom lens system and 2) locating the aperture stop at the image end of the zoom lens system. This arrangement is in itself advantageous for an instant photography camera because it provides an ideal location for the camera's shutter, namely, at the fixed location of the zoom lens system's aperture stop.
In addition to imposing the requirement that the exit pupil of a zoom lens system remains substantially fixed during zooming, instant photography cameras also impose the requirement that the f-number of the zoom lens system needs to vary in a controlled manner as zooming takes place. This requirement arises from the fact that in its preferred embodiments, the battery contained in the film pack of an instant photography camera provides all the power required for operation of the camera's various functions, including the power for the electronic flash. As a result, the amount of power allocated for the flash is limited by design. This, in turn, means that the amount of increase in the flash output available to compensate for decreases in the aperture of a zoom lens is also limited. This problem becomes more acute at the wide angle setting of the zoom lens, i.e., at the short focal length setting, since filling a wide angle with a flash requires more power than filling a narrow angle.
In general, a zoom lens system for an instant photography camera should have a f-number of about f/10 when the focal length is approximately equal to the film diagonal. For shorter focal lengths, the f-number should decrease, e.g., to approximately f/8. For longer focal lengths, a larger f-number, e.g., f/14, may be acceptable, especially if the camera's electronic flash has a variable angular output.
In addition to the above requirements, a zoom lens system for an instant photography camera should have: 1) a long back focal length sufficient for insertion of one mirror and in some cases three mirrors, e.g., a back focal length about 1.3 times the focal length at the wide angle position for a one mirror system and a back focal length about 1.7 times the focal length at the wide angle position for a three mirror system, 2) as short a front vertex to image distance as possible so as to minimize the storage size of the camera, and 3) small lens diameters in order to minimize weight and cost, e.g., for a 90 millimeter film diagonal, a maximum lens diameter of about 40 millimeters.
Instant photography cameras, however, do have certain characteristics which can be advantageously employed in designing a zoom lens system. In particular, due to the characteristics of instant photography color film, full color correction is in general not needed, e.g., a transverse chromatic aberration of approximately 0.125 millimeters can be tolerated.
In accordance with the invention, this characteristic allows for the use of lens units having strong optical powers and few optical elements. This combination can be employed in the zoom lens systems of the invention because the lens units need not be individually color corrected. Rather, color correction sufficient to satisfy the relatively relaxed requirements of instant photography color film can be achieved through the interaction of the units making up the zoom lens system, even though those units are not themselves individually color corrected.